Plate elements for fuel cell stacks

ABSTRACT

The invention relates to a plate element for a fuel cell stack which comprises a frame region and at least one inner region which is surrounded by the frame region. In addition, it has a plurality of webs which extend from the frame region into the at least one inner region and define a flow guidance structure in the at least one inner region, in which at least four bore holes are in the frame region of which at least two are connected with the flow guidance structure.

The invention relates to plate elements for fuel cell stacks in amonopolar or bipolar arrangement. In particular, the invention relatesto fluid-carrying electrical conductors and elements to be installedbetween the electrical conductors to insulate these electrically.

STATE OF THE ART

The differences between a bipolar arrangement and a monopolararrangement can be shown most easily using the equivalent circuitdiagrams that are provided in FIG. 1.

The bipolar arrangement corresponds to the upper part of the figure,which shows a series connection of individual voltage cells. The cellsare placed together in such a way that a plus pole and minus pole areopposite each other in each case, which allows a simple switching of thevoltage cells. This is the arrangement used, for example, to positionthe chambers of a lead accumulator, to insert the batteries into a torchor to position the fuel cell units in a bipolar stack.

The monopolar arrangement is that shown in the lower part of the figure.With this arrangement, the cells are placed together in such a way thattwo plus poles or two minus poles are opposite each other, in pairs,each time. In order to switch the individual voltage cells in serieshere, a comparatively complicated switching is necessary, and so such anarrangement of individual cells in most applications would not have anyadvantage over the bipolar arrangement. However, this does not apply forfuel cell stacks: advantages can also be achieved here with a monopolararrangement, as will be explained below referring to FIG. 2.

Within a bipolar stack, every cathode has two neighbouring anodes andmust be sealed against these to prevent fluid transfers. Within a cell,this is done in the interaction of electrolyte material (inside) andseals (outside). The separation between the neighbouring cells is madein each case through a separating plate, one side of which forms thecathode chamber and the other side of which forms the anode chamber. Thefluid passages in the separating plates are formed in such a way thatthe cathode fluid from the one side (cathode side) does not come intocontact with the anode fluid from the other side (anode side).

In a monopolar arrangement, on the other hand, anode and cathode pairsare formed. Indeed, the cathodes and anodes within such a pair must beelectrically insulated from each other, but the fluid regions themselvesdo not need to be separated as long as the electrical conductivity ofthe cathode and anode fluids remains negligible (which is generally thecase in spite of a certain conductivity, especially of the anode fluid).Two cathodes can therefore be grouped together in each case into onecathode chamber, and two anodes into one anode chamber.

FIG. 2 shows an exploded view of such an anode chamber. An arrangementof this type is described, for example, in DE 100 40 654 A1. Anelectrically insulating frame element 1 is sandwiched between twocurrent collectors 2. The current collectors in turn border onelectrolyte devices (MEA) (not shown in the figure). The currentcollectors 2 are used for current removal at the MEA-current collectorinterface, but at the same time should not noticeably reduce the contactsurface of the anode fluid with the MEA: for this reason, the innerregion of a current collector 2 is bridged with thin transverse webswhich are sufficiently wide and numerous for current removal, but are sonarrow that the active contact surface of the fluid with the MEA definedby the recesses 4 is not substantially reduced.

To guide the flow along the stack axis, four bore holes 5 are providedin the frame region (in the corners in this case) of the electricalconductors, whereby in each case two diametrically opposed bore holesserve to guide the anode fluid and the cathode fluid. Corresponding boreholes 6, 7 are also provided in the frame element 1. The bore holes 7are connected via openings with the inner region 8 of the frame element1. The supply of anode fluid into the inner region 8 is via one of thetwo bore holes 7, and the removal via the other, diametrically opposedbore hole 7. The inner region 8 represents the main volume of the anodechamber of the cell shown, since the thickness of the frame element 1 isfar greater than that of the current collector 2.

The statements made here for the anode chamber also applycorrespondingly for the cathode chamber. With a cathode chamber, thecentral frame element has recesses at the two other diametric fluidopenings. For this, the sketched type of frame elements 1 only needs tobe turned around, so that only one type of frame elements is necessaryfor the building of the stack.

In the arrangement shown, the flow through the corner regions, which donot exhibit any intake or outlet, is much less than, for example, thatthrough the central region of the anode chamber. In order to achieve amore even flow distribution within the chambers, net-like insertions(not shown in the figure) can be provided in the anode and cathodechambers. But even with these, the fluid exchange in the stagnationregions of the anode and cathode chambers is lower than along theintake-outlet section, so that the active surface available cannot beput to the best use. Stagnation regions of this type form in particularif the stack is not favourably positioned in terms of space.

DESCRIPTION OF THE INVENTION

In view of these defects, it is an object of the present invention toprovide improved stack elements which will overcome the disadvantagesdescribed above.

This object is solved by the plate element according to the inventionwith the features of claim 1 and the fuel cell stack assembly with thefeatures of claim 11.

Advantageous further developments are given in the sub-claims.Particularly preferred embodiments are described with reference to theattached figures.

The plate element according to the invention for a fuel cell stackcomprises a frame region and at least one inner region which issurrounded by the frame region, a plurality of webs which extend fromthe frame region into the at least one inner region and, in the at leastone inner region, define a flow guidance structure which is formed byrecesses between the webs, and at least four bore holes in the frameregion, at least two of which are connected with the flow field.

The term “plate element” aims to show that the item which is the objectof the claim has clearly smaller dimensions in one spatial directionthan in the two spatial directions at right angles to it. In the onespatial direction, the thickness of the plate elements can actually beso small that the plate element shows no intrinsic stability. The plateelement according to the invention therefore also aims to include foilswhich are structured in accordance with the features described above.

The term “flow guidance structure” is understood to mean recessesbetween the webs or at least a cohesive structure of recesses betweenthe webs which define a channel or a plurality of channels in the innerregion of the plate element. Each channel is (for example via openingsthrough the frame region) connected with two bore holes, of which one isfor the supply of fluid and the other for the removal of fluid. Throughthis at least one channel, the flow guidance structure is suitable forguiding (channelling) a fluid flow in the plane of the plate. Toactually accommodate a fluid and guide the flow, the flow guidancestructure naturally needs side restrictors which are provided byadjacent level surfaces of the adjoining stack elements (for example,those of the membrane electrode units).

To form several independent channels, the webs can be joined on bothsides with the frame region. But the webs can also be joined just on oneside with the frame region. In the simplest case, they have a straighttongue shape and run in a toothed arrangement, alternating from oppositesides of the frame region into the inner region of the plate element.Instead of straight tongues, more complex web shapes can also be used,such as “T”-shaped or “L”-shaped webs, which can also be positionedalternately to form a flow field.

The plate element is formed advantageously in such a way that it can beinstalled in different orientations at different positions of the fuelcell stack. The shape of the frame is therefore preferably square orrectangular. However, other shapes are also conceivable, which, if theyare turned by less than 360°, congruently turn over on themselves, i.e.in particular the shapes of an equilateral triangle or other regularpolygons, and also circles or ellipses.

The flow guidance structure may also be a plurality of channels runningnext to each other (for example, in parallel) within an inner region.Preferably, however, the flow guidance structure is formed, in an innerregion surrounded by the frame region, by a meandering flow channel or,correspondingly in the case of several inner regions, by a plurality ofsuch meandering flow channels. In this case, the entire inner region(except for the webs) can be evenly filled with fluids with the simplestpossible fluid supply and fluid removal.

The flow guiding properties of the plate elements according to theinvention mean that a locationally independent operation is possiblewithout any loss of performance, whilst with the arrangement sketched inFIG. 2, in which the flow is not guided, fluid is only removedsatisfactorily from out-of-the-way cathode or anode regions with thehelp of gravity. If used as a direct methanol fuel cell (DMFC), watercan collect, for example, in less favourable operating positions on thecathode side and stop the oxygen needed for the reaction from being ableto spread over the entire active surface. Secondly, the gaseous CO₂produced on the anode can also lead to the blockage of the activesurface of the anode if the gas cannot escape quickly upwards.

In a preferred embodiment, the plate element consists of a conductivematerial, such as graphite for example, but particularly preferably of ametal or a metallic compound. In this embodiment, the plate element canbe used as the electrical conductor of a cell unit. Metals or metalliccompounds are preferred since they are generally cheaper, easier to workand mechanically more stable and have a higher electrical conductivitythan non-metallic materials. Metallic electrical conductors are alsoeasier to contact electrically.

In a further preferred embodiment, the plate element consists of aninsulating material. In this embodiment, the plate element can be usedfor the electrical insulation of neighbouring electrical conductors inmonopolar arrangements.

In an advantageous further development, the plate element according tothe invention is made as a laminate of a conductive layer and aninsulating layer. If such laminates are used, the construction of thefuel cell stack is easier, since separate insulating units no longerneed to be used. The layer thicknesses of the conductive layer and aninsulating layer can be the same. The laminate can, however, also be aninsulator with a metal coating or a metal with an insulating coating inwhich the thickness of the coating is clearly less than the totalthickness.

The construction of monopolar fuel cell stacks can be further simplifiedif the plate element is formed as a laminate from an insulating layerand two conductive layers, which embed the insulating layer like asandwich. Here too, the various layer thicknesses are not basicallysubject to any limitations. If this further development is used,electrolyte units and plate elements are simply stacked alternately tobuild up the stack.

In a particularly preferred further development, the plate element hasat least one rib which extends from the frame region outwards. Via thisat least one rib, electrical conductors can be more easily linked witheach other electrically in a monopolar or bipolar arrangement. With theinsulating plate element, no such ribs are needed, but they may occurfor production reasons if the same device is used as for manufacturingthe electrical conductors and if they are not generally a disturbance.

In a further development which particularly simplifies the electricalswitching, ribs are provided on at least two opposite sides of the frameregion. In this case, one type of electrical conductor can easily beswitched in various orientations.

For monopolar arrangements in particular, a further development of theplate element is advantageous in which the ribs are offset against eachother on opposite sides of the frame region. As a result, the electricalconductors can be installed in such a way that its ribs with respect tothe ribs of the two adjoining electrical conductors in each case areoffset and are thus not a hindrance in the case of contacting with oneof the next-but-one electrical conductors.

The fuel cell stack assembly according to the invention comprises amembrane electrode unit (MEA) which is connected at least on one sidewith the conductive side of a plate element formed as an electricalconductor. Because of the connection, the fuel cell stack assembly has ahigher mechanical stability than the individual elements, making thebuilding of the stack easier. Moreover, if the fuel cell stack assemblyis prefabricated, a better electrical contact can, under somecircumstances, be achieved between the electrolyte device and theelectrical conductor than is possible if the electrical conductor plateelements and the electrolyte device are simply placed next to eachother.

In the following, the invention is described on the basis ofparticularly preferred embodiments and with reference to the attachedfigures.

The following are shown:

FIG. 1 shows the qualitative difference between a bipolar and amonopolar arrangement;

FIG. 2 shows the formation of a fluid chamber for a monopolararrangement according to the state of the art;

FIG. 3 shows a plate element in accordance with the present invention;

FIG. 4 shows a bipolar arrangement using plate elements according to theinvention;

FIG. 5 shows a monopolar arrangement using plate elements according tothe invention;

FIG. 6 shows a further monopolar arrangement using plate elementsaccording to the invention;

FIG. 7 shows a further preferred embodiment of the plate elementaccording to the invention as a current remover device in a bipolarstack;

FIG. 8 shows a fluid-insulating intermediate plate for the currentremover device shown in FIG. 7.

FIGS. 1 and 2 have already been described in the introduction.

FIG. 3 shows a preferred embodiment of the plate element 10 according tothe invention.

The plate element 10 can be made from an electrically conductivematerial (generally metal) and may be used in this embodiment as acurrent collector for bipolar or monopolar fuel cell stacks.

It has a circumferential frame region 11, whereby ribs 12 extend fromtwo opposite sides of the frame region 11 outwards and webs 13 inwards.The webs 13 do not extend as far as the opposite side of the frame andare arranged on an alternating basis so that a meandering channel 14 isdefined by the recesses between the webs 13. In the frame region 11,four bore holes 15, 16, 17, 18 are also provided, of which twodiametrically opposite bore holes 16 and 18 are connected via openings16 a and 18 a with the meandering channel 14.

When installed in a stack, part of the frame region 11 and the webs 13are in electrical contact with the MEA and are used to conduct power. Atthe same time, the webs 13 are also used for even fluid distributionacross the active surface and for fluid guidance across this surface.The fluid is fed via one of the two bore holes 16, 18, which areconnected via the openings with the meandering channel 14, flows throughthis channel 14 and is removed again via the other bore hole. Because ofthe guided flow, there are no stagnation regions with reduced fluidexchange, which gives greater efficiency. In addition, the flow guidanceguarantees locational dependence when the stack is used.

In an embodiment preferred for monopolar arrangements (but also usablefor bipolar arrangements), the ribs on the one side which extendoutwards are offset against the ribs on the other side in such a waythat with a juxtapositioning of the shown plate element 10 and a furtherplate element 10, which is rotated by 180° with respect to the x or yaxis indicated in the figure and positioned in such a way that the boreholes of the two plate elements are in line, the ribs of the two plateelements do not come into contact with each other.

For monopolar arrangements, it is moreover preferred that, in the eventof a rotation of the plate element 10 by 180° in the paper level, whenthe bore hole 17 lines up with bore hole 15 (and vice versa) and borehole 18 lines up with bore hole 16 (and vice verse), the ribs extendingoutwards from the one side do not cover the original position of theribs on the other side, but instead also are positioned between these.This means that it is possible only to use one type of electricalconductor to build up the entire monopolar stack.

The electrical conductor shown can be, for example, stamped from a metalplate or a metal sheet and can thus be made cheaply.

Electrically insulating intermediate plates for monopolar arrangementshave the same structure as the electrical conductor shown in FIG. 3 asregards the bore holes and the flow field. The ribs of the electricalconductors extending outwards are not necessary with the insulatingintermediate plates (see FIG. 5). However, it should be pointed out thatthese ribs would not be a hindrance with the insulating intermediateplates so that if necessary one and the same device can be used tomanufacture the electrical conductors and the insulating intermediateplates.

As an alternative to the insulating intermediate plate, in addition tothe pure metal plates, it is also possible to use metal plates coated onone side with an insulating material (or insulating plastic platescoated with a metal), which are structured as shown in FIG. 3. In thesecases, no separate seals for the mutual insulation of adjacentelectrical conductors need to be produced, which further cutsmanufacturing costs, simplifies assembly and further reduces the volumeof the stack (for more details, please see the description for FIG. 6).

FIG. 4 shows—in an exploded view and sectionally—a fuel cell stack in abipolar arrangement 100 with the electrical conductors of FIG. 3.

Of the four electrical conductors 110 a, 110 b, 110 c, 110 d, theelectrical conductors 110 a and 110 c on the one side, and theelectrical conductors 110 b and 110 d on the other side are identicallyaligned. If, without restriction of generality, it is assumed thatelectrical conductor 110 a is the cathode-side electrical conductor,then electrical conductor 110 b is an anode-side electrical conductor,electrical conductor 110 c is a cathode-side electrical conductor andelectrical conductor 110 d in turn is an anode-side electricalconductor. The electrical conductors are separated from each other byintermediate plates 120 or through electrolyte devices (MEA) 130. Thearrangement 120-110 b-130 defines the anode region of a cell unit,whilst arrangement 130-110 c-120 forms the cathode region of this cellunit.

The intermediate plates 120 are used for the separation of neighbouringfluid chambers, i.e. the separation of cathode and anode regions. Theymust prevent a mixture of cathode and anode fluids, but do not need tobe electrically insulating, so that, for example, they could alsoconsist of a thin metal foil.

As already mentioned, the arrangement 120-110 b-130-110 c-120 forms acell unit or—by analogy with FIG. 1—a voltage element. To switch thestack section shown—by analogy with the upper part of FIG. 1—the currentcollectors 110 a and 110 b, and the current collectors 110 c and 110 dare contacted with each other via the ribs provided for this purpose.The directly adjoining current collectors of various cell units shouldtherefore be preferably laid out here in such a way that these ribs arealigned with each other.

FIG. 5 shows—in an exploded view and sectionally—a fuel cell stack in amonopolar arrangement 200 with the electrical conductors of FIG. 3.

As with FIG. 4, this also shows four electrical conductors 210 a, 210 b,210 c and 210 d. If, without restriction of generality, it is assumedthat electrical conductors 210 a and 210 b are anode-side electricalconductors, then electrical conductors 210 c and 210 d are cathode-sideelectrical conductors.

The electrical conductors 210 b and 210 c are separated from each otherby an electrolyte/electrode device, such as a membrane electrode unit(MEA) 230. The arrangement 240-210 b-230 defines the anode region of acell unit, whilst the arrangement 230-210 c-250 forms the cathode regionof this cell unit.

The intermediate plates 240 and 250 are used for the mutual electricalinsulation of the electrical conductors 210 a and 210 b or 210 c and 210d. For this reason, the webs of the intermediate plates 240 and 250 mustbe exactly aligned with those of the adjoining electrical conductors.Anode regions or cathode regions adjoining each other in pairs form ineach case a common anode chamber or cathode chamber which are in eachcase closed on both sides by an electrolyte device 230.

The arrangement 240-210 b-230-210 c-250 forms a cell unit or—by analogywith FIG. 1—a voltage element. The switching is by analogy with thelower part of FIG. 1: to switch the stack section shown, for example,the anode-side current collector 210 b and the cathode-side currentcollector 210 d of the subsequent cell unit are electrically connectedwith each other (contacted) via the aligned ribs. The anode-side currentcollector 210 a is contacted with the next cathode-side currentcollector on the left (not shown), whilst the cathode-side currentcollector 210 d is contacted with the next anode-side current collectoron the right (not shown), and so on. This contacting in pairs can bemade simpler by ensuring that the ribs to be connected with each otherare aligned flush. The current collector elements are thereforepreferably made in such a way that the connecting ribs of each currentcollector are flush with those of the next-but-one current collector,whilst the connecting ribs of immediately adjacent current collectorsare offset (not-flush with each other). As shown in FIGS. 3 and 5, thiscan be achieved through suitable formation with a single type of currentcollector which is used each time in a different orientation.

As FIG. 6, which is based on FIG. 5, shows, a simplified construction ofa monopolar stack can be achieved if electrical conductors are usedwhich are formed as laminates, whereby one side is conductive and theother side is insulating.

For example, the electrical conductor shown in FIG. 3 can be coated onone side with an insulating material. In FIG. 6, the sides of electricalconductors 310 b and 310 d facing the viewer are to have a coating ofthis type. Through an alternating arrangement of such coated electricalconductors with uncoated electrical conductors in which the coated sideis always turned to the adjoining uncoated electrical conductors, amonopolar stack can be constructed without the use of separateinsulating intermediate plates, which represents a not inconsiderablesimplification in terms of construction. In addition, the insulatingcoating can be thinner than that of the insulating intermediate platesmade as separate units so that the total volume of the stack can bereduced.

It should be pointed out that in addition the sides of the electricalconductors 310 a and 310 c facing away from the viewer can also havesuch an insulating coating, even if this is basically not necessary. Inaddition to providing an even better guarantee of the electricalinsulation of adjacent cell units, this can also simplify the puttingtogether of the stack: adjacent electrical conductors must then alwaysbe put together in such a way that the fluid flow devices (bore holes,openings, channels) cover each other and the insulating layers areimmediately next to each other.

In principle, there are no preconditions as regards the thickness of theinsulating part and the conductive part of a laminate of this type. Ifthere are no other reasons (e.g. mechanical stability, electricalconductivity) to prevent it, an electrical conductor of this type cantherefore consist just as well essentially of insulating material whichis coated on one side with a conductive layer.

A further simplification of the construction of a monopolar stack can beachieved by laminates which have an insulating layer embedded betweentwo conductive layers (e.g. metal layers), i.e. which integrate, forexample, the plate elements 210 a, 240, 210 b of FIG. 4 in a plateelement. The stack can then be built up simply as an alternatingarrangement of laminates and electrolyte devices.

FIG. 7 shows a further preferred embodiment of the plate elementaccording to the invention and its alternating arrangement for theconstruction of a bipolar stack 400 (electrolyte devices andintermediate layers which separate neighbouring anode and cathoderegions from each other have been left out for the sake of simplicity).The plate elements 410 a and 410 b only differ in their orientation. Incomparison with similar structural features of the embodiment in FIG. 3,reference symbols increased by 400 have been used.

The dotted line of the plate element 410 a sketched on the top right ofthe figure divides the plate element into a frame region and an innerregion.

The frame region has a plurality of bore holes 415, 416, 417, 418, ofwhich the bore holes 416 and 418 are connected in pairs via openingswith recesses in the inner region of the plate element. All the boreholes are used for fluid guidance along the stack axis (at right anglesto the plate level). Bore holes 416 and 418 are also used for fluidguidance at right angles to the stack axis. For this, they are joined inpairs via openings with recesses 414 in the inner region of the plateelement, which define a plurality of parallel channels. These channels414 are separated from each other by webs 413 which are used for currentconduction with a plate element used as an electrical conductor (as isthe case with the bipolar arrangement). The webs are joined here withopposite frame regions. Between every pair of plate elements, either anelectrolyte device or a separating plate 420 is inserted, in the formand structure shown in FIG. 8. Bore holes 425, 426, 427, 428 in theouter region of the separating plate 420 (and also of the electrolytedevice, not shown) are aligned in the stack exactly with thecorresponding bore holes 415, 416, 417, 418 of the plate elements 410 a,410 b.

In the example in the drawing, for example, the plate elements 410 adefine anode chambers, whilst the plate element 410 b shows a cathodechamber. Bore holes 415 and 417 of the upper plate element 410 a areused in this case to guide the cathode fluid, whilst bore holes 416 and418 are used to guide the anode fluid and supply the anode fluid intothe anode chamber defined by the inner region.

The plate elements can be made from an electrically conductive materialand used in this form as current collectors for bipolar or monopolarfuel cell stacks. In contrast to FIG. 7, in the latter case the currentcollectors must always be aligned the same in pairs and neighbouringpairs turned in each case by 90°. If all the current collectors (or atleast one current collector of each pair) are provided with aninsulating coating on one side, then a monopolar arrangementcorresponding to FIG. 6 is possible. If the current collectors areconductive on both sides, electrically insulating plate elements withthe structure shown in FIG. 7 will be needed in addition in order toallow a construction of a monopolar arrangement corresponding to FIG. 5.The electrolyte device for the monopolar arrangement, on the other hand,is not different from that for the bipolar arrangement (cf. FIG. 8).

The present invention and its advantages have just been explained on thebasis of preferred embodiments. The scope of protection of the presentinvention, however, is defined solely by the following patent claims.

1. A plate element for a fuel cell stack, comprising: a frame region andat least one inner region which is enclosed by the frame region, aplurality of webs which extend from the frame region into the at leastone inner region and define, in the at least one inner region, a flowguidance structure which is formed by recesses between the webs, atleast four bore holes in the frame region, of which at least two areconnected with the flow guidance structure.
 2. A plate element accordingto claim 1 in which the flow guidance structure contains at least onemeandering flow channel.
 3. A plate element according to claim 1 whichconsists of a conductive material.
 4. A plate element according to claim3 which consists of a metal or a metallic compound.
 5. A plate elementaccording to claim 1 which consists of an insulating material.
 6. Aplate element according to claim 1 which is made as a laminate of aconductive layer and an insulating layer.
 7. A plate element accordingto claim 1 which is made as a laminate of an insulating layer and twoconductive layers which embed the insulating layer as if in a sandwich.8. A plate element according to claim 1 with at least one rib whichextends from the frame region outwards.
 9. A plate element according toclaim 8 with ribs on at least two opposite sides of the frame region.10. A plate element according to claim 9 in which the ribs arepositioned offset to each other on opposite sides of the frame region.11. A fuel cell stack assembly, comprising: a membrane electrode unitwhich is connected at least on one side with the conductive side of aplate element according to claim
 1. 12. A plate element according toclaim 2 which consists of a conductive material.
 13. A plate elementaccording to claim 2 which consists of an insulating material.
 14. Aplate element according to claim 2 which is made as a laminate of aconductive layer and an insulating layer.
 15. A plate element accordingto claim 2 which is made as a laminate of an insulating layer and twoconductive layers which embed the insulating layer as if in a sandwich.16. A plate element according to claim 2 with at least one rib whichextends from the frame region outwards.
 17. A plate element according toclaim 3 with at least one rib which extends from the frame regionoutwards.
 18. A plate element according to claim 5 with at least one ribwhich extends from the frame region outwards.
 19. A plate elementaccording to claim 6 with at least one rib which extends from the frameregion outwards.
 20. A plate element according to claim 7 with at leastone rib which extends from the frame region outwards.